1. Field of the Invention
The present invention relates to a light emitting device employing an element in which a luminescent material is sandwiched between electrodes (hereinafter referred to as a light emitting element), and in particular, a light emitting device employing a luminescent material in which EL (Electro Luminescence) is produced (hereinafter referred to as an EL material).
2. Description of the Related Art
Recently, a development of the light emitting device employing a light emitting element with an EL phenomenon has been progressed. Since the light emitting element itself has luminescent capability, a back light as used in a liquid crystal display device is not required. Further, since the light emitting device has a wide viewing angle, it is suitable for outdoor use.
As the light emitting device, there are two types, that is, a passive type (simple matrix type) and an active type (active matrix type), and both types have been greatly developed. In particular, currently, the active matrix light emitting device is noted. Also, as the EL material that becomes a light emitting layer of the light emitting element, there are an organic material and an inorganic material. Further, the organic material is classified into a low molecular system (monomer system) organic material and a polymer (polymer system) organic material. Both materials are greatly studied. The low molecular system organic material is formed mainly by evaporation and the polymer organic material is formed mainly by an applying method.
The organic material has a characteristic such that the luminescent efficiency is high and it can be driven with a low voltage compared with the inorganic material. Also, since it is an organic compound, various novel substances can be designed and produced. Thus, there is possibility that an element for emitting light with higher efficiency is discovered by the progress of a future material design.
When plural kinds of EL materials are laminated and thus functions of respective layers are separated from one another, the high efficiency of the light emitting element using the organic material is achieved. Generally, the light emitting element has a laminate structure as shown in FIG. 8. In this structure, light produced in the light emitting layer cannot be emitted unless the light is passed through an electrode. Generally, a transparent conductive film (typically, a compound of indium oxide and tin oxide, or the like) is used as an anode to emit light from the anode side. In addition, generally, a cathode in which a work function is small is not transparent. Thus, in order to emit light from the upper side of a substrate, for example, a treatment is required such that the cathode is thinned. Generally, when the emitted light is passed through the electrode, brightness of the light emitting element is decreased.
Generally, the light emitting element has the laminate structure as shown in FIG. 8. When such a laminate structure is obtained, light produced in the light emitting layer is cannot be emitted from the surface of the substrate or the surface of a sealing material unless the light is passed through the anode or the cathode. When light is emitted from the light emitting element, generally, the transparent conductive film (compound of indium oxide and tin oxide) is used as the anode to emit light from the lower side of the substrate. In addition, in the light emitting element, although the cathode in which a work function is small is used, its substance is not generally transparent. Thus, when light is emitted from the upper side of the substrate, for example, a treatment is required such that the cathode is thinned. Generally, when the light produced in the light emitting layer is passed through the electrode, external quantum efficiency of the light emitting element is decreased.